The present invention relates to a method of checking the workmanship of paint coating, such as primer coating, done to the peripheral areas of the windshield, rear window glass, etc. of an automobile or the coupling flanges, etc. of an automobile chassis.
Prior to glazing a window in the process of assembling an automobile, it is most common to apply a sealant to the whole periphery of each window glass such as a windshield and a rear window glass. Prior to applying the sealant, it is also common to apply a black primer to the window frame and to the portion of each window glass where the sealant is to be applied. The aim of the black primer coat is to obtain a reliably sealed condition.
Various manipulators are available for the automatic application of the sealant and the black primer.
The fluidity and surface condition of these paints remarkably differ with ambient temperatures and with whether or not these paints are in touch with air. The workmanship of paint coating is affected by the paint discharge rate and paint coating speed. It is comparatively difficult, therefore, to quantitatively control the paint discharge rate and uniformly apply a paint to the surface of an object to be coated therewith.
For this reason, the workmanship of paint coating is not always found to be normal, but is occasionally found to be defective. For example, lack of hiding occurs when the primer film thickness is insufficient, and stringiness occurs when the quantity of the primer is insufficient.
It is known to use a reflection type photoelectric sensor in a method of checking the workmanship of paint coating. In this known method, a checkup is carried out on whether or not the intensity of light reflected by the surface of a coat of paint falls within a preset level.
The trouble is that the workmanship of paint coating cannot always be checked accurately by this known method. Whether or not it can be checked accurately by this known method depends on the intrinsic condition of the surface of an object to be coated with the paint.
The criterion for judging the workmanship of, e.g., primer coating as coming up to the standard consists in whether or not the coat of primer has a prescribed width.
From this criterion, the following trouble results: If stringiness or lack of hiding occurs in some portions of the coat of primer, the surface of the object coated with the primer is exposed in these portions. Consequently, even if the coats of primer have an equal width, the intensity of reflected light incident on a photoelectric sensor differs with the colors of other intrinsic conditions of the surfaces of objects.
From the foregoing, it will be apparent that the reason for the workmanship of primer coating being not always checked accurately by the above-described conventional method is that, according to the difference in the intrinsic conditions of the surfaces of objects, the conventional method may possibly pass different judgments on two primer coatings applied respectively to the surfaces of two objects, even if these two primer coatings are of the same workmanship according to the above-mentioned criterion.
To put it concrete, let it be supposed that a black primer is applied to a white surface and a blue surface, that these two primer coatings are of the same workmanship, and that lack of hiding occurs in the same manner in these two coats of primer. Then the light reflected by the portions of the white surface in which the lack of hiding has occurred is stronger than the light reflected by the portions of the blue surface in which the lack of hiding has likewise occurred. Thus the intensity of reflected light incident on a photoelectric sensor differs with the intrinsic conditions of the surfaces of objects.
If the above-mentioned two coats of black primer, which are of the same workmanship, are subjected to a judgment on the acceptability on the basis of the same preset level, the black primer applied to the white surface will be judged to be unacceptable, while the black primer applied to the blue surface will be judged to be acceptable.